1. Field of the Invention:
This invention relates to a process for the preparation of sodium carbonate. More particularly this invention relates to a process for the preparation of sodium carbonate through wet calcination of a solid material containing sodium bicarbonate, sodium sesquicarbonate or a mixture thereof by direct contact with a hot, water-immiscible liquid, thereby liberating carbon dioxide and water and converting the sodium bicarbonate and sodium sesquicarbonate to sodium carbonate.
2. Prior Art:
Sodium carbonate, immensely consumed in many industries, is obtained mostly by the pyrolysis of sodium bicarbonate or sodium sesquicarbonate prepared by the ammonia soda process or ammonium chloride soda process. In the ammonia soda process, carbon dioxide gas is passed through ammonia-saturated salt-water to precipitate sodium bicarbonate, followed by separation of the precipitate and washing. This crude sodium bicarbonate is calcined to give sodium carbonate as represented by the following equation: EQU 2NaHCO.sub.3 +heat.fwdarw.Na.sub.2 CO.sub.3 +CO.sub.2 .uparw.+H.sub.2 O
Conventional dry calcination procedures suffer from a number of inherent disadvantages. For example, presently practiced methods for debicarbonation of sodium bicarbonate and sodium sesquicarbonate are based on dry calcination in fired or steam heated calciners and require substantially more thermal energy than the thermodynamic minimum for the reaction. Most of the energy consumed in these processes is not converted into useful work of the chemical transformation, but rather is dissipated as the essentially unrecoverable heat content of the gases leaving the calciner. Equipment limitations effectively preclude operation of these calciners at elevated pressures which would facilitate recovery of energy in a useful form, as for example steam, from the hot gases.
Wet calcination schemes have been proposed for the conversion of sodium bicarbonate, sodium sesquicarbonate and mixtures thereof into sodium carbonate which employ an aqueous liquor as the suspending and heat transfer medium. Illustrative of such wet calcination processes is the process described in U.S. Pat. No. 4,252,781. In the wet calcination process of that patent, a thick suspension of sodium carbonate anhydride is obtained by contacting high pressure steam and a suspension of sodium bicarbonate or sodium sesquicarbonate in a concentrated solution of sodium carbonate in a counter current fashion. The decomposition reaction is pushed almost to completion by maintaining the reaction temperature above 150.degree. C. which increases the concentration of bicarbonate ions in the solution. The sodium carbonate anhydride suspension is cooled which converts the sodium carbonate anhydride into sodium carbonate monohydrate. The crystalline monohydrate is separated from the mother liquor. The mother liquor is recycled for the preparation of the sodium bicarbonate or sodium sesquicarbonate suspension. The sodium carbonate monohydrate is calcined to provide dense soda ash of high purity and of substantially uniform particle size.
This known wet calcination process suffers from a number of inherent disadvantages. For example, aqueous bicarbonate and sesquicarbonate liquors are quite coorosive to carbon steel at elevated temperatures e.g., 100.degree. C. to 200.degree. C. This necessitates the use of costly corrosion resistant alloys in the fabrication of process equipment which contact these corrosive aqueous process streams. A further disadvantage of this prior art wet process results from the use of countercalcination current columns for debicarbonation of the sodium bicarbonate or sesquicarbonate. In order for such columns to function successfully, suspended solids must be in a finely ground state. This requirement greatly increases process cost and expense in that feed material originating from trona mining operations are usually coarse solids, and therefore must be processed further before use in the prior art process. Still another disadvantage of this prior art aqueous calcination process is that the process requires substantial residence time and a number of stages to effect reasonably complete decomposition of the sodium bicarbonate and sodium sesquicarbonate. This is apparently the result of thermodynamic equilibrium and kinetic limitations associated with the presence of an aqueous phase.